What we basically tried to do is explain what is meant by that term, putting it in a historical context and explaining the role that experiments with ultra-cold atoms, as opposed to “traditional” materials, can play in disentangling the mysteries. We placed a lot of emphasis on the model that John Hubbard (pictured) invented here, in the Harwell campus, in the early sixties, and on neutron scattering, which was responsible for the genesis of that model and is still producing many of the main puzzles today. Polite comments and enlightening criticism are certainly most welcome.